Mr Trudeau will have to defend his record, including his handling this year of an ethics crisis that proved politically costly.

The prime minister was found by a federal ethics watchdog to have improperly tried to influence a former minister over the prosecution of SNC-Lavalin, a major engineering and construction company facing a criminal trial.

That affair was raised on Wednesday morning by Conservative leader Andrew Scheer, who issued a rebuke against the Liberal leader, saying Canadians "cannot trust Justin Trudeau".

Can Trudeau recreate the magic?

Jessica Murphy, BBC News, Toronto

Justin Trudeau swept to power four years ago on a surge of support that coalesced around the Liberal Party, ending nearly a decade of Conservative rule.

His decisive victory was helped by many first-time and young voters who cast ballots for the Liberals, excited by Mr Trudeau's leadership, the positive tone of his campaign and the possibility of change.

Now the question is whether Mr Trudeau can conjure the campaign magic that led to that historic victory.

The young, progressive leader had an exceptionally long political honeymoon with the Canadian public.

He followed through on some major commitments - legalising recreational cannabis, championing gender equality, opening Canada's doors to Syrian refugees.

But he has struggled in recent months following a series of political fumbles and an ethics scandal.

And he has made decisions that have frustrated parts of his progressive base, including buying a major oil pipeline to ensure its expansion and scrapping a promise to reform Canada's electoral system.

The campaign is currently shaping up to be a battle between two parties: the Liberals and the Conservatives.

National polls indicate they are in a dead heat, with both on just over 30% support.

Mr Trudeau has been working hard to draw a clear contrast between the two parties, and he has faced some accusations of negative campaigning.

"I do not engage in personal attacks but I will be very, very sharp on distinctions around policy," he said on Wednesday.

The NDP are polling nationally in third place, on roughly 13%-14% support, with a slight lead over the Green Party.

Who's in the running?

Mr Trudeau was an untested leader in 2015 when he won a majority government. Now, it's his turn to face opponents who are unproven campaigners.

His main rival is the Conservative Party's Andrew Scheer.

Mr Scheer, 40, formerly the youngest-ever speaker in Canada's House of Commons, took the helm of the right-leaning party in 2017.

Another fresh face in federal politics is Jagmeet Singh, 40.

The leader of the left-leaning New Democrats (NDP), he is the first person of colour to have taken the helm of a major federal political party in Canada.

The separatist Bloc Quebecois, under new leader Yves-Francois Blanchet, will be competing for seats against the Liberals, Conservatives and the NDP in Quebec, a crucial battleground province in federal politics.

The Bloc only runs candidates in that province.

The Green Party, led by Elizabeth May, will be focusing efforts on trying to break through in British Columbia and Atlantic Canada.